Electric power distribution in a passenger aircraft is often very complex, requiring both alternating current (AC) and direct current (DC) power at varying levels. This power is generated during flight as the product of one or more generators tied to the output of the aircraft propulsion system. On the ground, however, the aircraft electrical requirements are supported by ground power units that couple to the aircraft electrical system.
A typical aircraft electrical distribution system includes both three-phase and single-phase loads. The system circuitry usually omits any safety ground conductor in order to save weight in the aircraft. Additionally, the skin of the aircraft often serves as part of the aircraft's neutral voltage reference point. During ground power servicing, the aircraft's neutral voltage is carried through a ground power harness and tied to the ground power unit's neutral point. The neutral conductor, between the aircraft neutral and the GPU neutral, carries any current caused by imbalance on the single-phase aircraft loads.
Unfortunately, continuity between the aircraft and GPU neutral conductors occasionally breaks, through wire fatigue or the like. As a result, the aircraft loses the neutral reference point for all AC loads that use a neutral reference. Consequently, without a neutral connection, any wye-connected loads often become unbalanced. In essence, the neutral point and the phase-to-neutral voltages move around according to the load impedances.
For aircraft power systems faced with a broken neutral conductor, the unbalanced condition will exist for virtually any load condition. Further, if the coupled ground power unit does not provide over-voltage protection circuitry, unacceptably high AC supply voltage may be applied to some of the phases, subjecting sensitive electronic instrumentation to potentially damaging voltage levels. Moreover, passengers and personnel loading and exiting the aircraft may experience an electrical shock should both the earth (GPU neutral) and the aircraft skin (aircraft neutral) be touched simultaneously due to the potential difference generated between the respective neutral points on account of the broken neutral connection.
To protect individuals and sensitive electronic equipment from the hazards often associated with ground fault and over-voltage conditions, those skilled in the art have devised many different forms of ground fault protection. Generally, the systems monitor the magnitude of the current flowing from main conductors to earth instead of returning back through the neutral or other phase conductor. Once the monitored current exceeds a threshold level, power to the main conductors is interrupted.
One general proposal for carrying out ground fault protection involves placing a current transformer over all of the main conductors except the safety ground wire. The transformer includes a burden resistor to detect current proportional to the ground fault current. During normal operation, the instantaneous sum of the currents through the main conductors is zero. In a ground fault situation, current flows back through a ground connection which does not cancel the current flowing out. As a result, a voltage is developed across the transformer burden resistor that is indicative of the ground fault current magnitude. If the fault current exceeds a predetermined threshold, such as approximately 0.005 amperes, a safety disconnect is tripped, shutting off power to the circuit. This circuit is typical of a conventional ground fault circuit interrupter (GFCI or GFI) commonly used for residential outlet applications.
A second method of effecting ground fault protection is similar to the previously described technique, but involves inserting a current transformer over the ground wire. Current flowing from the electrical unit back through ground can be detected, and if the detected current exceeds thresholds of between 5 to 1000 amperes, to cause a power interruption. This type of circuit is commonly implemented in high power industrial applications carried out in controlled environments with highly trained personnel.
While the above-described transformer techniques are believed adequate for their specifically designed applications, several problems can occur if implemented in an aircraft ground power system. For example, the conventional current transformer systems often cannot detect the high voltage on an aircraft in most situations. This is because large ground currents (measured by the transformers) flow only if a skin-to-ground connection is made. If no skin-to-ground connection is established, then the high phase voltages can still remain unseen on the aircraft electrical system with the potentially "shocking" difference in potential existing between the aircraft and the bridge.
Other techniques of attempting ground fault protection for aircraft ground power systems include building an over-voltage circuit into the GPU itself to detect ground faults at the GPU. Unfortunately, this method fails to detect any problem resulting from a broken neutral connection because the voltage sensed at the GPU is sampled before the broken neutral connection. Moreover, attempts at remote voltage sensing, wherein the voltages on the three-phase conductors at the aircraft are brought back through the GPU harness to the GPU, have been attempted. However, this method ignores the neutral connection and the problems associated with a broken neutral conductor.
What is needed and heretofore unavailable is a ground fault and over-voltage detector and method for detecting the loss of a neutral connection between an aircraft power system and a ground power unit. The detector and method of the present invention solves this problem.